Nowadays, with increasing awareness of environmental protection, more and more products are designed in views of low power consumption. Since most power consumed by an incandescent bulb is radiated as heat, rather than as visible light, the incandescent bulb has very low luminous efficiency. Mercury vapor lamps are relatively efficient but they must be carefully handled and properly disposed because the released mercury vapor often contaminates the air we breathe. For environmental friendliness, the incandescent bulbs and the mercury vapor lamps are gradually replaced by light emitting diodes (LEDs) in many applications.
A light emitting diode is a semiconductor diode capable of emitting light with high luminous efficiency. Different from a common incandescent bulb, the light emitting diode has the advantages of small size, no heat radiation, low power consumption, long service life, and quick response speed. For fabricating a light emitting diode, a semiconductor substrate is doped with a variety of dopants to improve optical properties. By changing the concentration of the dopants or the packaging structure, an intended single color of the light emitted by the light emitting diode is achieved.
Typically, LED is used as a light source. In some cases, for expanding the applications of the LED, it is desirable to change the color of light to meet the practical requirement.
U.S. Pat. No. 6,357,889 disclosed a color tunable light source that contains multiple light emitting diodes (LEDs) with different emission wavelengths and multiple phosphors with different excitation and emission wavelengths. The phosphors can absorb the light emitted by the light emitting diodes and release light of different wavelengths from those of the absorbed light. By variably controlling power supplied to these light emitting diodes, light of different wavelengths are emitted. Through color mixing, the color of the light emitted by the light source is varied.
U.S. Pat. No. 6,636,003 disclosed a color temperature adjustable lighting device that contains multiple colored LEDs and multiple white LEDs with different arrangement. By variably controlling drive currents supplied to these LEDs, light of different intensities are emitted. The colored light emitted from the colored LEDs are combined with the white light emitted from the white LEDs to produce resultant light having a desired color temperature adjustable white light.
As previously described, since multiple LEDs are necessary to adjust the color or the color temperature of the lighting device, the control circuitry becomes very complicated. In addition, the tunable color or the color temperature is usually restricted to a narrow extent.
Therefore, there is a need of providing a lighting device for producing light with adjustable color and color temperature by using a single light source.